A digital image used to process an image as digital data can be easily copied by a computer or the like and transmitted via a communication line without degrading the image quality, compared to a conventional analog image. This feature, however, makes it easy to illicitly copy and redistribute a digital image having a copyright or the like.
To prevent this, there is known a digital watermark method. Digital watermarks are roughly classified into an invisible digital watermark obtained by invisibly embedding watermark information such as copyright information or user information, and a visible digital watermark obtained by positively visibly forming in an image a watermark image such as the logotype of a company having a copyright.
As for the invisible digital watermark, embedded watermark information cannot be recognized or is hardly recognized in an embedded image at a glance. Watermark information is rarely deleted, but is illicitly copied and distributed more frequently than visible watermark information. Even if a digital image is illicitly copied or distributed, watermark information remains in the digital image. An illicit user can be specified by a user ID or the like embedded as the watermark information.
As for the visible digital watermark, watermark information is visibly written in a digital image. It is difficult to directly utilize the digital image, suppressing illicit copying and illicit distribution. As a conventional visible digital watermark embedding method, the pixel value of an image representing copyright information such as the logotype of a copyright holder is replaced with the pixel value of an original image, embedding copyright information in the original image. The drawback of this method is that the original image cannot be reconstructed without difference information because the pixel value of the original image is lost.
In the conventional visible digital watermark embedding method, a replaced pixel value must be acquired again in reconstructing an original image. This substantially means reacquisition of the original image, increasing key information.
Japanese Patent Laid-Open No. 2000-184173 proposes a method of embedding visible watermark image shape information in an image by arithmetic processing (encryption) between all pixels or some bits and an embedding serial sequence at a position where a visible digital watermark is to be embedded. This method can implement a completely reversible visible digital watermark. However, this reference does not fully consider the image quality.
Japanese Patent Laid-Open No. 8-256321 proposes a method of extracting part of the bit string of an image which is compression-coded by JPEG or MPEG compression coding, directly converting the extracted bit string by an independently defined conversion method without referring to a part other than the extracted part, and decoding the image. This technique is called “semi-disclosure”, and can provide image information to the user while controlling the image quality of the disclosed image information.
This method preserves the feature of an original image, but is a kind of scramble (encryption). This method does not fully consider the image quality of an image containing a digital watermark.
Japanese Patent Laid-Open No. 8-241403 proposes a visible digital watermark method which considers the image quality. In this reference, an input image is converted into a uniform color space. A linear luminance value is scaled in accordance with the watermark intensity (initial scale coefficient) or noise at a position where a visible digital watermark is to be embedded. As a result, the luminance is increased/decreased to embed a visible digital watermark. This reference will achieve good image quality, but does not describe any method of an implementing a completely reversible watermark.